The present invention relates to a method and apparatus for controlling the gaps between lips of a die for extruding a molten plastic in an apparatus for producing a plastic sheet-like material such as, for example, a plastic film.
More in particular, the invention relates to a method and an apparatus for operating the die adjusting bolts while taking into account the interaction between separate adjustments that are individually made.
Normally, in an apparatus for producing a plastic sheet-like material such as a plastic film, as shown in FIGS. 1a and b a die having a plurality of bolts 1 for controlling the gap between the lips is used. A molten plastic is introduced into the die through an inlet 2 by a pump (not shown), and is extruded through a gap between a pair of die lips 3, and is solidified to form a sheet-like material. The thickness of the so-produced plastic sheet is greatly influenced by the gap between these die lips 3. Especially, the distribution or pattern of thickness in a transverse direction on the sheet may be said to be determined by the distribution of the gap between die lips, across the die.
Accordingly, the distribution of thickness of a plastic sheet-like material is controlled by finely adjusting this distribution of the gap between the die lips, by means of adjusting bolts. The distribution of the gap between die lips is controlled by rotating the adjusting bolts 1 and selectively pressing or pulling the lips 3 together or apart. However, because the nozzle lips 3 are made in one piece, and are made of metal, when one bolt is moved, the gap between the die lips changes extensively in other positions as shown in FIG. 2, the gap between the die lips at the positions of other bolts being influenced by the adjustment. Because of this interaction, it is very difficult to adjust the lips 3 to bring about the desired gap distribution, and adjustments for controlling the gap between the die lips are at present carried out on a trial-and-error basis and depend upon the experience of a skilled worker, requiring a very long period of time. Especially, when these operations are carried out while the products are being produced, the percentage of products coming up to the standard of quality is reduced greatly, due to the unevenness of thickness of the product across its width.